Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 0
Due after one year through five years, Amortized Cost	3,247
Due after five years through ten years, Amortized Cost	25,771
Due after ten years, Amortized Cost	297,043
Totals, Amortized Cost	326,061
Due in one year, Estimated Fair Value	0
Due after one year through five years, Estimated Fair Value	3,375
Due after five years through ten years, Estimated Fair Value	26,952
Due after ten years, Estimated Fair Value	299,034
Totals, Estimated Fair Value	329,361
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	0
Due after five years through ten years, Amortized Cost, Held to Maturity	1,139
Due after ten years, Amortized Cost, Held to Maturity	749,912
Totals, Amortized Cost, Held to Maturity	751,051	$ 676,426
Due in one year, Estimated Fair Value, Held to Maturity	0
Due after one year through five years, Estimated Fair Value, Held to Maturity	0
Due after five years through ten years, Estimated Fair Value, Held to Maturity	1,147
Due after ten years, Estimated Fair Value, Held to Maturity	764,732
Estimated Fair Value, Held to Maturity	$ 765,879	$ 688,779